Revenue recognition - Costs to obtain a contract (Details) $ in Thousands	6 Months Ended
Jun. 30, 2020 USD ($)
Change in the balance of costs to obtain a contract
Balance at beginning of period	$ 335
Deferral of costs to obtain a contract	230
Recognition of costs to obtain a contract	(303)
Balance at end of period	$ 262

Amount, after accumulated amortization and accumulated impairment loss, of asset recognized from cost incurred to obtain or fulfill contract with customer.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 340
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=68051541&loc=SL49131252-203054

Name:	us-gaap_CapitalizedContractCostNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant